Buy both. Left 4 Dead 2 cannot fully replace Left 4 Dead. #2 may have more content and more activity, but #1 has better atmosphere, better animations, and you can actually see your body if you have the settings on high. Left 4 Dead 2 may be the game you play more, but Left 4 Dead is an experience you shouldn't deny yourself. Especially when it's so cheap right now.

I'd get both. I like L4D1 much more than LFD2. Try them out...well worth thw money.